Ultra High Speed Digital Imaging

The purpose of image sensors is to reproduce, as faithfully as possible, the image of an illuminated object or a light source, as formed by a suitable optical system. In the specific context of ultra-high-speed digital imaging, an additional constraint is the need to digitize the received image in very short time frames. The priority is therefore to have highly “sensitive” electronic sensors in order to achieve a satisfactory signal-to-noise ratio during short exposure times and to be able to “refresh” the image as quickly as possible to capture the next view on the image sensor. For extremely short exposure times, on the order of nanoseconds, it is still necessary to place light amplifiers in front of the semiconductor sensor. This often results in a decrease in the spatial resolution of the images, which is directly related to the time savings achieved. The high-speed sensor is therefore primarily characterized by its acquisition rate in frames per second (fps). Among the main types of semiconductors used, we will focus primarily on describing the two major families of optoelectronic sensors used today: CCD sensors and CMOS sensors, and their variants:

• CCD (Charge-Coupled Device), which collects, transfers, and converts the electrical charge generated by incident photons;

• CMOS (Complementary Metal-Oxide Semiconductor), which performs charge collection and conversion at the site where the charge generated by incident photons is collected.

Pushing the boundaries of high-speed imaging (sensitivity, the resolution-frequency trade-off, and the total number of images captured) is an ongoing challenge for designers, who must compete with ingenious technological solutions to improve performance. Over the past 15 years, two major categories of high-speed cameras have emerged: CMOS cameras and on-chip storage CCD cameras. While the former represent pure technological advancements in standard CMOS sensors coupled with rapid transfer techniques to an onboard high-bandwidth internal memory, the latter are constantly improving and enable the very rapid acquisition and transfer of an image to a storage site that is geographically very close—in the sense of the electronic chip—to its collection point (the active image). Thus, hybrid techniques using processes similar to CMOS operation are implemented within CCD technology. The only limitation lies in the speed at which electrons move within the collection and storage sites.

These cameras must meet a specific set of technical specifications, currently defined by users and manufacturers. They are described, in particular, by the European Machine Vision Association under the designation EMVA1288. We will discuss various aspects related to...